Disk recorders, whether magnetic or optical, are used extensively for storing computer-generated data. Such recorders, having storage capacities in the hundreds of megabytes and beyond, combined with relatively fast data access rates, are especially useful for storing large amounts of computer-generated data requiring repeated accesses. Optical drives, though having generally slower data access rates than their magnetic counterparts, offer higher areal storage densities.
Optical media is of three general types, media which can be read only (ROM), media which can be written only once, and media which can be written, erased, and written again. Write-once media (WORM) is permanently altered when write power levels are produced by the laser beam. Erasable media, such as magneto-optic (MO) media, is not permanently altered when data is written. In the MO media, the magnetic orientation of reactive material therein is altered in the writing process, and in the erasing process, the magnetic orientation is reordered.
In operating an optical disk system, it is necessary to identify the particular sector and track upon which the laser beam is directed. That identification information is included in a sector header stamped onto the disk itself. The user area, that is, the data area, may be either write-once or erasable, but the sector header will always be permanent.
Optical disk drives make use of optical media that react to bursts of light, such as may be produced by the rapid switching of a semiconductor laser. In order to write data on optical media, the laser power must be controlled at a relatively high power level, that is, above a Curie temperature, so that the media can be altered in accordance with an applied directional vertical magnetization which direction corresponds with the desired digits of data to be written thereon. To read data from the optical media, the laser power level is reduced to a level such that the optical media is not altered by the laser beam, but rather a light reflected off the optical media indicates the presence or absence of media alterations, that is, digits of data.
When reading data from MO media, the remnant magnetization of one or the other polarity rotates the linear polarization of a light beam reflected off of the MO media, wherein the light beam is rotated according to the Kerr rotation effect which rotation depends upon the direction of the vertical magnetization. The reflected light is then converted into polarized light creating a p+s and a p-s polarization component. By detecting the p+s and p-s polarization components of the light beam, an MO data signal is generated.
The readback signal is detected differentially for the best signal to noise ratio. Each orthagonal component of the polarized light, p+s and p-s, is focused on a separate photodetector. The MO signal is derived by taking the difference of current or voltage signals generated by the polarized p+s and p-s light signals impinging on each photo detector. One technique for generating the difference signal is to amplify each photocurrent by a current or voltage amplifier, as appropriate, and then determine the difference for MO signal detection, whereby common signal components cancel (e.g. noise components). Thus, substantially only the Kerr rotational signal remains for providing a reading of the stored digital data.
Two highly competitive areas of magneto-optic disk drive design include: increasing areal data density; and improving data access speed. Data density has been improved, in part, by using light having shorter wavelengths (for example, using light in the blue range versus infrared light), and hence decreasing the spot size of the light, used for reading and writing data. Reductions in the light wavelengths may also provide increased data access speeds for a given rotational speed of the MO media since a shorter distance must now be traveled before reaching the next bit of data. However, decreased spot size further strains the optical reading mechanisms such that it becomes difficult to provide a sufficient signal to noise ratio. For example, the optical reading mechanisms typically make use of a half and/or a quarter waveplate that must be accurately adjusted, usually robotically during manufacturing, to properly balance the p+s and p-s light components while still providing an adequate signal to noise ratio to the optical detectors.
Still further increases in speed can be obtained by increasing the rotational speed of the MO media. As continued increases in speed are accomplished, new error components are introduced by the amplifiers used to amplify the voltage or current signals produced by the photodetectors. Such error components include undesirable high frequency errors due to mismatched amplifiers for the p+s and p-s polarizations, respectively. Although it is known to low-pass filter the output signals of the photodetectors to compensate for low frequency noise components, high frequency errors remain uncompensated. At higher frequencies, device mismatches (due to parasitic capacitances and resistances, etc.) which heretofore could be substantially ignored, now present a dominating mismatch factor since further degradations in the signal to noise ratio occur. Such parasitics can introduce an additional +/- five percent error, which combined with the optical errors, may be unacceptable. Simply adjusting the waveplates, therefore, may not be adequate compensation. Even after making adjustments, aging and/or environmental effects can cause further misbalances due to shifts in alignment or parasitic effects.
Accordingly it is desired to provide an apparatus for making adjustments in the p+s and p-s polarization amplifiers to compensate for both optical and electrical effects both in manufacturing and in the field.